Experimental evidence for previously unclassified calcium phosphate structures in the casein micelle.

1H-31P Cross-polarization magic angle spinning (CP-MAS) measurements of 40-d-old Mozzarella cheese and 20 mM EDTA-treated casein micelles revealed that each sample had immobile phosphorus with the same spectral pattern, which did not match that of native casein micelles. To identify the immobile phosphorus bodies, 1H-31P CP-MAS spectra and cross-polarization kinetics measurements were undertaken on native casein micelles, EDTA-chelated casein micelles, and reference samples of ß-casein and hydroxyapatite. The results showed that the immobile phosphorus bodies in the mature Mozzarella cheese had the following characteristics: they are immobile phosphoserine residues (not colloidal calcium phosphate); they are not the product of phosphoserine to colloidal calcium phosphate bridging; the phosphate is complexed to calcium; their rigidity is localized to a phosphorus site; their rigidity and bond coupling is unaffected by protein hydration; and the immobile bodies share a narrow range of bond orientations. Combining these observations, the best explanation of the immobile phosphorus bodies is that bonding structures of phosphorus-containing groups and calcium exist within the casein micelle that are not yet clearly classified in the literature. The best candidate is a calcium-bridged phosphoserine-to-phosphoserine linkage, either intra- or inter-protein.

An NMR study of the freezing of emulsion-containing drops.

Various nuclear magnetic resonance (NMR) techniques were used to monitor the freezing behaviour of suspended 2-mm-diameter drops. The drops were composed of hydrocarbon oils emulsified in either water or water/sucrose mixtures. As such they were good model systems for the study of spray freezing, sharing structural similarities with potential products such as ice cream. In particular, simple 1H NMR spectroscopy was used to monitor and individually quantify the freezing or solidification behaviour of the various constituent species of the drops. In addition, the effect of freezing on the emulsion droplet size distribution (and hence emulsion stability) was also measured based on NMR self-diffusion measurements. The effect of freeze/thaw cycling was also similarly studied. The nucleation temperature of the emulsion droplets was found to depend on the emulsion droplet size distribution: the smaller the droplets, the lower the nucleation temperature. Emulsion droplet sizing indicated that oil-in-sucrose-solution emulsions were more stable, showing minimal coalescence, whereas oil-in-water emulsions showed significant coalescence during freezing and freeze/thaw cycling.